FIG. 1 illustrates a prior art fluorescent light fixture. The fixture has a sheet metal chassis 10 and two fluorescent lamps 12 and 14. The lamps are connected in series and powered by a ballast 16. The main AC power out of the ballast is applied across the red and blue leads, while the yellow lead provides a center connection. In some ballasts, there may be only one of each color lead, while in others, there may be two leads of each color to provide power for heating filaments at each end of the lamp.
Because the lamps are connected in series, all of the current flowing through one lamp nominally flows through the other lamp, and therefore, the two lamps should appear to have the same intensity. Various factors, however, may cause different amounts of current to flow through each lamp. For example, fluorescent lamps tend to be sensitive to metallic objects located close to the lamp. If the sheet metal chassis 10, which may include a reflector, a ballast cover, etc., is slightly wavy or has a dent as shown at 18, or is otherwise closer to one lamp than the other, it may cause a current imbalance. As another example, the relative lamp currents may be affected by differences in wiring impedance caused by the routing of wire leads within the fixture. This is illustrated in FIG. 1 where an excess portion of the blue lead is arranged loosely in the fixture, whereas the excess portion of the red lead is tightly coiled in a manner that may give the red lead a higher inductance than the blue lead. An additional source of current imbalance is manufacturing tolerances of the lamps which may cause different impedances, current requirements, etc.
When operating at moderate to high power levels, current imbalances caused by these factors tend to be less noticeable because the leakage or unbalanced currents are relatively small compared to normal lamp operating currents. For example, if two series-connected lamps are operating at a few hundred milliamps, a few milliamps of imbalance is unlikely to cause a perceptible difference in the relative intensity of the two lamps. As the ballast power is reduced, and the lamps are dimmed to a lower brightness level, the current imbalance may become more pronounced, and one lamp may appear significantly brighter than the other, especially at the lowest dimming levels.